1. Field of the Invention
The present invention relates to an optical recording method which makes high velocity recording possible to an optical recording medium having a recording layer containing an organic colorant and which supplements a deficiency in recording sensibility to realize good recording/readout characteristics.
2. Discussion of Background
In recent years, there has been a progress in shortening of the wavelength of a laser beam to be used for recording/readout, in order to improve the recording density of an optical recording medium. Namely, the research and developments have been advanced from an optical recording medium which can be recorded and readout by a laser beam with a wavelength of about 780 nm or 830 nm as currently commonly employed, to DVD employing a semiconductor laser beam with a wavelength of from 640 nm to 680 nm, or further to an optical recording medium which can be recorded and readout by a laser beam of 600 nm or less.
Such an optical recording medium may be of a write-once type on which recording is possible only once, or of a rewritable type on which recording/erasing can be repeated. For example, as a write-once type, an organic colorant type optical recording medium containing an organic colorant in its recording layer may be mentioned, and as a rewritable type, a magneto-optical recording medium employing photomagnetic effects, or a phase-change type optical recording medium utilizing a change in reflectance caused by a reversible change between an amorphous state and a crystalline state, may, for example, be mentioned.
With respect to an organic colorant type optical recording medium, standards have recently been established for 3.95 GB capacity of a write-once type optical recording medium (DVD-R) on which recording/readout is carried out by a laser beam with a wavelength in the vicinity of 640 nm. As shown partially in such written standards, a method has been established to control the timing of the edges of recorded marks in mark length modulation recording, by dividing the incident laser beam for optical recording into multi-pulses. Also with respect to a recording apparatus, an optimum system for high density recording has been practically developed.
For example, FIG. 27 in Tech. Rep. IEICE CPM 96-152 (1997) discloses a case wherein, at a recording linear velocity of about 3.5 m/s, the length of the forefront pulse of the recording beam is adjusted to be from 1.3T to 1.5T (wherein T is the reference clock period) and the second and subsequent pulses (multi-pulses) are adjusted to be from 0.6T to 0.7T.
While there is a recording medium having a very small recording linear velocity dependency of recording characteristics (hereinafter referred to as xe2x80x9crecording linear velocity dependencyxe2x80x9d), like a phase-change type optical recording medium, an organic colorant type optical recording medium has a large recording linear velocity dependency and thus is barred from high speed recording.
For example, when recording is carried out on a GeSbTe type phase-change type optical recording medium (hereinafter sometimes referred to simply as a xe2x80x9cphase change mediumxe2x80x9d) by adopting one type of xe2x80x9cpulse strategy for recordingxe2x80x9d (hereinafter referred to simply as a xe2x80x9cpulse strategyxe2x80x9d) at a recording linear velocity of 1.4 m/s, 5.6 m/s or 10 m/s, the optimum recording power at each velocity is about 11 mW, 12 mW or 13 mW, respectively, and thus the difference in the recording sensitivity is small.
On the other hand, in the case of an organic colorant type optical recording medium such as DVD-R, if recording is carried out at each of the above velocities by employing the conventional pulse strategy (the basic strategy in 3.95 GB written standards for DVD-R), the optimum recording power is 3 mW, 12 mW or 15 mW or higher. Thus, the change in recording sensitivity to the recording velocity is very large.
FIG. 1 shows the optimum recording powers when recording was carried out at a 1-time velocity (recording linear velocity: 3.5 m/s) and at a 2-times velocity (recording linear velocity: 7.0 m/s) using a recording beam with a wavelength of 635 nm employing the same pulse strategy (the basic strategy for 3.95 GB written standards for DVD-R) on ten types of DVD-R differing in the colorant contained in the recording layer. From this Figure, it is evident that the recording linear velocity dependency of the organic colorant type optical recording media does not depend on the type of the colorant.
The optical recording laser has been improved to a substantial extent, but a laser of a high power is not yet widely employed. In a practical recording apparatus, the recording power is at a level of about 15 mW at the maximum, in many cases, when a laser having a wavelength of from 600 to 700 nm is employed. Further, a recording laser of from 400 nm to 500 nm which has recently been commercialized, has a limit at a level of 14 mW at present. Under these circumstances, such a large recording linear velocity dependency is a serious problem for the organic colorant type optical recording medium.
As observed in the case of a semiconductor laser having a wavelength of 780 nm which is used for recording/readout of CD-R, it may be possible that improvement of the power of a semiconductor laser can be realized in a very short period of time. However, it is expected that even if in a near future a semiconductor laser of high power at a level of at least 20 mW with e.g. a wavelength of from 600 to 700 nm will be practically developed, recording at a higher velocity at a level of 21.0 m/s (corresponding to 6-times velocity in the DVD type optical recording medium) or 28.0 m/s (corresponding to 8-times velocity in the DVD type optical recording medium) may not be accomplished merely by the improvement of the recording power.
Generally, there are two methods for solving the above problems.
Firstly, a method of inventing a pulse strategy i.e. setting of pulses or pulse trains for emission of a recording laser beam to be used for recording, may be mentioned. Such a method is disclosed, for example, in Electronic Materials (1996) June edition, p 50, 3.95 GB written standards for DVD-R (ver. 1.0), and JP-A-11-195242. In these publications, a pulse strategy wherein the pulse division into (nxe2x88x922) pulses is the base, and the forefront pulse is from 1.2T to 1.5T, and multi-pulses are from 0.6T to 0.7T, and a recording linear velocity of 3.5 m/s (1-time velocity recording), are employed. The recording system employing this pulse pattern is not sufficient to overcome the recording linear velocity dependency, although it is extremely effective to overcome another problem of the organic colorant type optical recording medium i.e. to reduce the difference in recording sensitivity between a long mark and a short mark caused by the small thermal conductivity of the colorant layer (recording layer).
Another method for solving the problem of the recording linear velocity dependency is to select the construction of the organic colorant type optical recording medium, for example, to select the colorant or the material for the reflective layer.
In the case of the organic colorant type optical recording medium, the thermal conductivity of the colorant contained in the recording layer, itself, is small, whereby, as compared with an inorganic type recording layer of e.g. a phase-change medium, the remaining heat effect in the scanning direction by the recording laser beam, is small. Further, usually, a metal reflective layer of high thermal conductivity is laminated, whereby cooling by heat dissipation in the direction from the recording layer to the reflective layer, is substantial. For these two reasons, from its construction, the recording linear velocity dependency is large as compared with e.g. the phase-change medium.
The following measure is, for example, conceivable to overcome such a drawback:
1) a metal having a smaller thermal conductivity than gold (among the metal reflective layers practically used for optical recording media at present, the thermal conductivity is smallest) is used as a reflective layer, and further, the thickness of such a reflective layer is made thinner than 60 nm which is considered to be the critical thickness for a metal layer practically serving as a reflective layer);
2) the colorant contained in the recording layer is changed to an organic colorant having a high absorbance at the recording wavelength, i.e. changed to a recording layer having a larger extinction coefficient at a recording/readout wavelength; or
3) an interlayer having a low thermal conductivity, of e.g. a nitride, is provided between the metal reflective layer and the recording layer.
However, in each case, the change is to such a direction that the reflectivity of the optical recording medium becomes substantially lower than the conventional organic colorant type optical recording media (CD-R, DVD-R). It is not desirable to overcome the recording linear velocity dependency by such a method, if the interchangeability with a ROM medium as one of substantial merits of the current organic colorant type optical recording media, i.e. interchangeability of CD-R with CD or interchangeability of DVD-R with DVD, is regarded to be of importance.
If diversification of products will advance, and interchangeability with a ROM medium will not be so important, and a demand for an organic colorant type optical recording medium of low reflectivity type will be created, the above-mentioned measures 1) to 3) to overcome the recording linear velocity dependency, will not be particularly problematic.
The present inventors have studied the conditions for application time of a laser for recoding, which are effective to supplement a deficiency in recording sensitivity during high velocity recording on a usual organic colorant type optical recording medium and to supplement a deficiency in sensitivity of an organic colorant type optical recording medium having a relatively poor recording sensitivity. At the same time, they have studied how to control adverse effects such as a deterioration in C/N of wobble (an increase of noise) due to a substantial increase of modulation by high velocity recording, and a deterioration in the quality of address signals of e.g. LPP (landprepits) after the recording.
As a result, they considered it necessary to adjust the time for the forefront pulse in the pulse train to be longer than ever with a laser beam divided into pulses to be irradiated for recording and to adjust also multi-pulses to be longer, to take a thermal balance in the longitudinal direction of recorded marks.
Namely, the present invention provides an optical recording method for an optical recording medium having a recording layer containing an organic colorant on a transparent substrate, which comprises applying a recording laser beam divided into nxe2x88x922 (wherein n is an integer of at least 2) pulses (or into 1 pulse when n is 2), to the optical recording medium, to form a mark having a recording data of nT (wherein T is the reference clock period), wherein the length of the forefront pulse is at least 1.7T and at most 2.4T, and the lengths of the second and subsequent pulses (multi-pulses) are at least 0.7T and less than 1.0T.
Further, the present invention provides an optical recording medium on which recording can be made by such an optical recording method, and an optical recording medium having recording made by such an optical recording method.